Option to give a time code where to reset synchronisation

First of all thank you so much for this gem.

Let's suppose I have a video with some section in the middle that was not present in the video the subtitle was made for, or vice-versa (previously section, video credits, commercial breaks, etc).

It could be practical to have an option (for instance --reset-at) give a time where to reset synchronization, so the sync process is cut in two parts that are merged together.

This option could be allowed to be used several times if necessary.